Traveling wave tube oscillators



April 1, 1958 J. H. BRYANT TRAVELING WAVE TUBE OSCILLATORS 2 Sheets-Sheet 1 Filed Oct. 7, \1953 tgJOwofP/W) i /\1 Canfora/Puf@ COLLECTOR /0 e 7o E LOAD OUTPUT NEA/VJ' /NPUT MEA/VS 5 EZECTQCW GUN 7 INVENTOR JO/1W BRYANT ATTORN EY .April 1 1958 v l J, H. BRYANT v 2,829,252

TRAVEL-ING WAVE TUBE OSCILLATORS ATTORNEY TRAVELING WAVE TUBE OSCILLATORS John H. Bryant, Nutley, N. J., assignor to International Telephone and Telegraph Corporation, Nutley, N. J.,

a corporation of Maryland Application October 7, 1953, Serial No. 384,553

4 Claims. v(Cl. Z50-36) This invention relates to traveling wave tube oscillators yand more particularly to helical transmission line phase shifting devices incorporated in the feedback circuit thereof.

The traveling wave type of tube is particularly useful in wide band microwave systems since it is capable of amplifying radio frequency energy over an unusually wide band of frequencies. The tube includes a form of propagating structure, usually a helix, for transmission of a microwave energy for interaction with an electron beam closely associated with the propagating structure. The helical characteristic of the propagating structure is such that the axial velocity of microwave signals conducted along the helical path is approximately the same as or slightly slower than the velocity of the electrons of the beam whereby the electrical field of the microwave signals interact with the electron beam for Iamplilication of microwave frequency electromagnetic energy.

As is known an electronic oscillator includes ingeneral two fundamental portions. The first portion provides for a net transfer of D. C. kinetic energy into radio frequency energy, or in other words, is capable of amplifying a signal at radio frequencies. The second portion provides a feedback of a stipulated amount of the output power to the input of the amplifier-having the proper frequency and appropriate phase relationship with the energypat the input to the amplifier to sustain oscillations.

Therefore, given an amplifier of the traveling Wave tube type, itis then only necessary to provide a feedback arrangement to realize an oscillator capable of prov'iding oscillatory energy in UHF and microwave frequency range and over a wide band of frequencies. The employment of a traveling wave tube amplifier as a high frequency oscillator has been recognized by those skilled in the art as evidenced by the patent to Field #2,603,773 and the patent to Doehler #2,580,007 which generically illustrate `a phase shifting element of both coaxial and waveguide transmission types for achieving the appropriate phase relationship between the feedback energy and the energy at the input to the traveling wave tube.

.While the prior art cited above illustrate in a broad sense a vfeedback loop in conjunction witha traveling wave tube having phase shifting'means for achievement of oscillatory action, it is an object of this invention to provide in conjunction with a traveling wave tube amplier a lfeedback path including specific novel phase shifting devices other than conventional waveguide and coaxial transmission line for achieving the proper phase relationship of the feedback energy with respect to the inergy input to the traveling vwave amplifier enabling the generation of high frequency oscillations.

Another object of this-invention is to provide a traveling wave tubeV oscillator including in the feedback loop thereof a phase shifting element employing as the activel portion thereof a helical transmission line device.

AI feature of this invention is the provision of a negative *dispersive helical structurel asvv the phase shifting ice element capable of automatically maintaining exactly the same total number of wavelengths around the oscillatory loop including the feedback path and traveling wave ampliiier. The phase shifting element of this embodiment may comprise a passive negative dispersive helix with suitable R. F. input and output terminals, or may comprise an active traveling wave amplifier including therein as the propagating structure a negative dispersive helix.

Another feature of this invention is the provision of a phase shifting element including a slotted coaxial line with an inner conductor having a helical conguration and a probe inserted through a longitudinal slot in the outer conductor of said coaxial line to enable the desired phase adjustment of the feedback energy.

A further feature of this invention is the provision of a coaxial line with an inner conductor having a helical configuration and a probe inserted through a longitudinal slot in the outer conductor of said coaxial line in coupled relation Lwith said inner conductor disposed such that the extremities of said inner conductor are connected to couple the output energy from the output means of the traveling wave tube oscillator to the'load thereof with said probe coupling a given amount of said output energy to the feedback path and simultaneously adjusting the phase angle thereof for sustaining oscillation within a traveling wave tube oscillator.

Still another feature of this invention is the provision of a phase shifting element including an electron beam tube having an interaction region, an electron guny disposed 'at one end thereof providing a stream of electrons for passage through said interaction region, and a collector electrode disposed at the other end of said interaction region wherein said interaction' region vincluding a first helix adjacent said electron gun associated with the output circuit of the traveling wave tube, a second helix adjacent ,said collector associated with the input section of said traveling wave tube, a drift tube intermediate said tirst and second helical'structures. The drift .tube of this electron beam tube is provided with a variable direct current potential which controls the transit time of the electron beam through said drift tube for adjusting the phase angle of the feedback energy to sustain oscillations within said traveling wave tube.

The above-mentioned and other features and objects of this invention willbecome more apparent by reference to the following description taken in conjunction `with the accompanying drawings, in which: t

Fig. l is a diagrammatic illustration of a known Fig. 5 diagrammatically illustrates an electron beam-` tube having a variable transit time drift tube disposed centrally upon the axis of said electron beam tube in association with helical input and output propagating structure.

Referring to Fig. l, there is illustrated therein a diagrammatic representation of a traveling wave electron discharge device 1 in conjunction with energy output means 2 having therein a power dividing arrangement for coupling a predetermined amount of the output .i energy through phase shiftingelement and'tilter'com-i Patented Aprrl, 1958.

ponent 4 to the input means 5 of the interaction section 6 of traveling wave electron discharge device 1.

The traveling wave electron discharge device 1 adopted for use in hyper and ultra-highfrequencies comprise as an essential part thereof anelectron gun portion 7 for developing a beam ofA electrons for passage through the interaction section 6 in an axial relation to the wave propagating structure 8 illustrated to be a helical transmission line wherein the desired interaction between the wave energy and the electron beam takes place. In order to perform the amplification through interaction the dimension of the helix 8 must be such that the energy waves will be propagated in a mode having electric field components in the direction of the electron beam and at a .speed along the axis of the helix of an order to which the electron beam may be accelerated by the use of moderate voltages. For accelerating voltages in the order of 1500 to 2000 volts, the helix may be wound `with several turns-of wavelengths along the axis. The interaction between the electron beam and the traveling energy `wave is` accumulative and to insure an adequate interaction, the axial length of the helix should be of the order of 30 to 40 wavelengths. To assure the ow of theelectron beam from gun 7 it is necessary to provide an electron beam focusing means such as magnet Q disposed coaxially about electron discharge device 1. Magnet 9 may `be of the well-known solenoid type employed in conjunction with traveling wave electron discharge devices to focus the electron beam for travel along the axis of electron discharge device 1, or may be of the permanent magnet type as described in copending applications of J. H. Bryant and H.W.Y Cole, Serial No. 321,342, 'led November 19,1952, now abandoned, and J. H. Bryant, Serial No.` 318,060, filed October 31, 1952, both being entitled, Traveling :Wave Electron Discharge Devices."

lIn employing `electron discharge device 1, or types related thereto, as an oscillator itis necessary, as in all oscillators, to provide a means for adjusting the phase angle of ther energy coupled along `the feedback path of the oscillatory circuit to sustain oscillations in device 1. At any givenfrequency it can be assumed that there are N wavelengths between input means S and output means 2. If the output means 2 includinga power divider is connected to input means by `a length of radio frequency transmission line there willbe a certain length of transmission line for which the phase is correct for sustaining oscillations 'within device `1. lflthe frequency within device 1` is changed, however, the number of wavelengths N will change, and the electrical length of the feedback radio `frequency transmission line will likewise change causing the phase of the feedback `energy to become incorrect for sustaining oscillations unless some special provision is made to provide a suitable phase shift in the feedback line.

The propagating structure normally yemployed within a traveling wave tube has `a dispersive characteristic wherein the velocity of propagation-increases with decreasing frequency. Such a dispersive characteristic may normally be achieved by ahelical structure in free space, surrounded coaxially by a uniformally conducted cylinder, or surrounded `by a uniform dielectric constant medium. The same `dispersive characteristics may likewise be achieved by the helical structure `surrounding a uniformally coaxial cylinder and also by a helix disposed between uniformally conducting coaxial cylinders. lt is this characteristic of `a helical propagating structure which causes the wavelengths to change about an oscillatory loop when the frequency of oscillation is altered.

In accordance with the principles of the present invention it has been discovered that a phase shifting element having a dispersive characteristic which causes the velocity of propagation for electromagnetic waves to decrease as the` frequency .is decreased will maintain the wavelengths about an oscillatory loopat a constant value as the frequency is changed. A means for obtaining negative dispersion employing a helical propagating structure, for example, may include the` disposing of dielectric material coaxial with the helical structure having a predetermined radius with respect to the radius of the helix. The action here is that as the frequency is decreased the radio frequency fields extend out further from the helix and a greater percentage of the radio frequency energy travels thru the dielectric. With a proper choice of the inner radius for the dielectric material, and with respect to the helical radius, and the proper choice of dielectric constant for the dielectric material, negative dispersion will be obtained. A second means for obtaining the desired negative dispersion characteristic may include the placing ol' a helical structure inside and coaxial to an outer conducting shield which has the properties of conducting only in the longitudinal or axial direction.

Referring to Figs. 2 and 3, there is illustrated therein two embodiments of phase shifting elements utilizing the negative dispersive `characteristic of a helical structure which cooperates in an oscillatory loop to control thc phase angle of the feedback energy to maintain the wavelengths about the loop constant thereby sustaining oscillations within the traveling wave tube 1. With particular reference to Fig. 2 there is illustrated therein a passive helical structure 11 having appropriate input and output radio frequency means 12 and 13, respectively. The action of this passive helix 11 is such that as the frequeucy is decreased and the number of wavelengths within the ,traveling wave tube 1 decreases, the number of wavelengths between input and output means 12 and 13 of the phase shifter 3 increases due to the negative dispersive characteristic and thereby maintains the same total number of wavelengths around the oscillatory loop, said oscillatory loop being defined by traveling wave tube 1, output means 2, phase shifter 3a, liltcr 4 and input means 5.

Fig. 3 illustrates a phase shifting element 3b as including ran active traveling: wave tube 14 having an electron gun 15 for producing a beam of electrons for passage thru the propagating structure or interaction region 16 for collection by collector 17. The path of the electron beam emitting from gun 15 is focused and maintained in a clearly defined path by means of a magnetic eld produced by magnet 18 of the solenoid type or permanent magnetic type dependent upon the tube employed. The input means 19 of traveling wave4 tube 14 is connected to the output means 2 of traveling wave tube 1 for coupling the radio frequency feedback energy to the propagating structure 16 andhence to the output means 20 for coupling to the feedback filter 4. The propagating structure 16 disposed within traveling wave tube 14 has a negative dispersive characteristic such that `a change of frequency within traveling wave tube 1 causingthe number of wavelengths therein to decrease will be compensated by the increase of wavelengths within traveling wave tube 14 and, therefore,` will sustain the oscillations within traveling wave tube `1.

`Referring to Fig. 4, another embodiment of the phase shifter is illustrated `at 3c to control the phase angle of the feedback energy coupled along the feedback path from output means 2 to input .means 5 of tube 1. By controlling the phase angle in the feedback path the wave lengths of the oscillatory loop are maintained constant andthus oscillations will be sustained. A phase shifter of this embodiment comprises a coaxial line 21 having an outer` conductor 22 and an inner conductor 23 separated therein by a dielectric material. Inner conductor 23 has a helical configuration, one end` of which is coupled to the power divider of output means 2. Outer conductor 22 of coaxial line 21 has disposed thereiua slot 24 which enables the movement and positioning of a probe 25 along the inner conductor 23. The wavelengths of the feedback loop are maintained constant when `the frequencyis changed by positioning-the probe at an appropriate -pointalong the length of inner conductor 23 of .coaxial line 21 thereby increasing or decreasing the number of wavelengths in the feedback path to compensate for the increase or decrease of wavelengths within-traveling wave Vtube 1 upon change .of frequency therein. l

yReferring to Fig. 4A, a modification of the phase shifting element Vembodiment of Fig. 4 is illustrated at 3d. Thearrangement villustrated provides a means for obtaining'the dual functions of powerdividing and phase shifting by single piece of apparatus in-a'ccordance with the principles of this inventionto couple a given lamount of output power into the feedback path and to control the phase angle ofthe energy coupled along the feedback path from output means 2a` to the input means` of tube 1. The output means 2a of this embodiment consists of a waveguide coupler properly connected to helix 8 for removing wave energy therefrom. The inner conductor 23a of element 3d is connected thereto for conduction of the wave energy to a given load. Probe 25a extends through slot 24a-of outer conductor 22a for coupling to inner conductor-23a such that a given amount of the output energy may be coupled therefrom for conduction along the feedback path. The coupling between probe 25a and conductor 23a may be a capacitive or inductive coupling, or may be by direct contact as illustrated, the tightness of the coupling determining the amount of output energy extracted therefrom. The positioning of probe 25a longitudinally along conductor 23a will control the phase angle of the energy fed back and as before will compensate for an increase or decrease of wavelengths in tube 1 upon change of frequency therein. Thus, the phase shifting element 3c of Fig. 4 may be utilized to control the phase angle of feedback energy only, or may be utilized as a power divider and a phase shifting element dependent upon the manner of associating this element with the output energy of tube 1 and the load to which this energy is coupled.

A further embodiment of the phase shifting element is illustrated at 3f in Fig. 5 as including an electron beam tube 26 having an electron gun 27 for production of an electron beam for travel through an interaction region 23 to a collector 29. The interaction region 28 includes therein a helical propagating structure 30 at the input portion of tube 26 which is coupled at one end to the power divider of output means 2, a helical propagating structure 31 disposed in the output portion of tube 26, one end of which is coupled to filter 4. Intermediate the propagating structures 30 and 31 there is disposed a drift tube 32 exhibiting a variable transit time to the electron beam passing therethrough and carrying thereon the feedback energy. The variable properties of drift tube 32 are madel possible by the coupling of variable D. C. potential to drift tube 32 from power supply 33 illustrated as a battery employing thereon a sliding contact 34.

Battery 33 is also illustrated as supplying the appropriate potential to the elements comprising the electron beam tube 26, with the propagating structures 30 and 31 being disposed at ground potential from a D. C. point of view as indicated by radio frequency chokes incorporated in the circuitry thereof.

The action of the variable transit time drift tube 32 is to slow down or speed up the ow of feedback energy carried on the electron beam traveling therethrough to increase or decrease the wavelengths therein'to compensate for the change of wavelengths resulting from a change of frequency within traveling wave tube 1. The control of the transit time withindrift tube 32 is accomplished by movement of arm 34 upon the power supply 33.

While the above described embodiments of the phase shifting element have been related to a traveling wave tube amplifier including a helical propagating structure, it will be obvious to those skilled in the art that the principles and structure of the yphase shifting elements herein disclosed will function with other types of traveling wave tube amplifiers having propagating structures other than helical in form. f

While I have described above the principles of my invention in connection-with specic apparatus, it is to be clearly understood that this description is made only by way ofy example and not as a limitation to the scope of my invention as set forth in the objects thereof and in 'the accompanying claims.

I claim:

1. In a radio frequency wave oscillator, an oscillatory loop comprising a traveling wave amplifier including -means for propagating radio frequency Waves along a given axial path, said means having wave energy input and output sections, an electron gun adapted to project an-electron beam along said path for reaction with the wave energy propagated therealong, and feedback means Icoupled to said output section for selecting a predetermined amount and frequency from the output energy for coupling back to said input section, said feedback means having therein a helical transmission line type of phase shifting element'for adjusting the phase angle of said selected output energy to sustain oscillations within said amplifier, said phase shifting element comprising an inner conductor, an outer conductor coaxial of said inner conductor, a dielectric medium separating said conductors, said inner conductor having helical configuration and said outer conductor having a longitudinal slot therein, and a probe coupled to said input section disposed to move within said slot in coupled relation to said inner conductor for control of the phase angle of said selected output energy.

2. kIn a radio frequency wave oscillator, an oscillatory loop comprising a traveling wave amplifier including means for propagating radio frequency waves along a given axial path, said means having Wave energy input and output sections, an electron gun adapted to project an electron beam along said path for reaction with the wave energy propagated therealong, and feedback means rcoupled to said output section for selecting a predetermined amount and frequency from the output energy for coupling back to said input section, said feedback means having therein a helical transmission line type of phase shifting element for adjusting the phase angle of said selected output energy to sustain oscillations Within said amplifier said feedback means including an inner conductor, an outer conductor spaced about and coaxially of said inner conductor, said inner conductor having a helical conguration coupled at one extremity thereof to said output section, the other extremity of said inner conductor serving as the output terminal for said oscillator, and said outer conductor having a longitudinal slot therein, and a probe coupled to said input section disposed to move within said slot in a coupled relation t0 said inner conductor for selecting a predetermined amount of the oscillator output energy for couplingback to said input sectionl and to simultaneously control the phase angle of said selected output energy.

3. An electronic oscillator comprising a traveling wave amplifier including an elongated helical propagating structure for propagating electrical waves therealong having electrical wave input and output sections, an'electron gun for providing a stream of electrons suitable for travel lengthwise of and in the field region of said propagating structure for interaction with the energy of the electric waves thereon, and an oscillatory feedback` means coupled from said outputy section to said input section including a power dividing arrangement for extraction of a predetermined amount of feedback energy from said output section, a frequency responsive filter to assure that said feedback energy has a predetermined frequency, and a helical transmission line phase shifting element for adjusting the phase of said feedback energy for sustaining J oscillations within said traveling wave ampliiier, ksaid phase shifting element comprising `a coaxialtline including anninner conductoncoupled to said power dividing element, a movable probe coupled to saidf input section, and an outer conductor coaxial of said inner'conductor, said `inner conductor `havinga helical configuration and said outer conductor having a longitudinalslot therein, said probebeing disposed for movement Within `said slot and in coupling relation to said inner conductor for control of the phase angle of said feedback energy.

4. Anrelectronic oscillator comprising a traveling wave amplifier including an elongated helical' propagating structure for propagating electrical waves therealong having electrical Wave input and output sections, an electron gun for providing a stream of electrons suitablev for travel 1 lengthwise of and in the field region of said propagating structure for interaction with `the energy of the electric waves thereon,` and an oscillatory feedback means coupled from said output sectionto said input section n cluding a power dividing arrangement forextracton of 20 2,611,101

a predetermined amount of feedback energy from said output section, a frequency responsive filter to assure that 8 said feedback 'energy has a `predetermined frequency, and a helical transmission line; phase shifting element for adjusting the phase of said feedback energy for sustain ing oscillations within said traveling wave amplifier,` said feedbackmeans including an innericondu'ctor `coupled to said output section for conduction of the energy output of said oscillator, a movable probe coupled to said input section, and an outer conductor coaxial of said inner conductor, said inner conductor having a helical configuration and said outer conductor having a longitudinal slot therein, said probe being disposed for movement Within said slot and in coupling relation to said inner conductor for selecting a predetermined amount of feedback energy from said output energy and to simultaneously control the phase angle of said feedback energy.

References Cited in `the-file of this patent UNITEDSTATES PATENTS Wallauschek Sept. 16, 1952` 2,636,148 Gorham Apr. 21, 1953 2,653,270 Kompfner Sept. 22, 1953 

